Michael J. Barden is a senior hydrogeologist for Hydro Geo Chem in Tucson with over 25 years of environmental experience as a consultant and state regulator. He has dealt extensively with hydrogeological evaluation, contaminant transport and fate evaluation, and vadose zone and groundwater modeling and has expertise in risk assessment, environmental geochemistry and natural attenuation of organic and inorganic contaminants. He was a member of a National Academy of Science committee on intrinsic remediation evaluating the scientific basis for natural attenuation and has been an instructor for numerous training courses on monitored natural attenuation, risk assessment, and modeling.